1. Field of the Invention
The present invention concerns a servo control of a disk drive, and more particularly a technique for controlling a position error signal for securing a servo control of high reliability.
2. Description of the Related Art
Generally a hard disk drive is widely used as a auxiliary memory for writing data transmitted from a host computer and transmitting data to a host computer by reading it. A servo control consists of a track search mode for moving a head to a target track to write and read data by detecting servo information in a disk and a track following mode for positioning a head on a center line of a target track.
In an earlier hard disk drive, a disk is rotated by a spindle motor. A head is positioned above the surface of the magnetic disk and installed on a arm extended vertically from an arm assembly of a voice coil motor. A preamplifier preamplifies a signal picked up by the headduring reading and drives the head for writing encoded write data sent from a read/write channel unit onto a magnetic disk during writing. The read/write channel unit is connected to the preamplifier, A/D converter and a disk data controller and supplies read data to the disk data controller by detecting a data pulse from a signal preamplifier in the preamplifier and encoding and decoding it in order and write data sent from the disk data controller to the preamplifier by encoding it. The read/write channel unit supplies an analog reading signal to the A/D converter by detecting it from a preamplifier signal from the preamplifier.
The A/D converter, connected to the read/write channel unit, receives an analog reading servo reading signal and supplies it to a microcontroller by converting the analog reading signal to position error signal digital data and also supplies an output current from a voice coil motor driver to the microcontroller by converting it into a digital signal. The microcontroller, connected to the disk data controller, controls track detection and track following. The voice coil motor driver, connected to the voice coil motor and the D/A converter, controls the operation of the voice coil motor by control of the microcontroller.
The voice coil motor moves the head on the magnetic disk in accordance with the direction and level of the driving current supplied from the voice coil motor driver.
It is essential to confirm the position of the head in advance of writing and reading data onto the hard disk drive. The microcontroller moves the head to a target track by a preprogrammed servo control algorithm and it further controls the head to be positioned on the center line of the track for reading and writing data after moving the head to a target track. For servo control, a track's own information, "gray code data" and comparison information, "burst data", which are normally called servo information, are written on each track of the disk by its manufacturer.
Two bursts are written on each track in advance as part of the servo information. An A burst and an B burst are written on adjacent tracks alternately, wherein each burst is written on one side of a center line of a track in one track and positioned continuously in the direction of the circumference without overlapping each other. These A and B bursts are detected by the head.
The position error signal is defined to be the difference between the detected A burst detection level and the detected B burst detection level.
The microcontroller controls the head to follow the center line of the track by using the position error signal corresponding to a change of its position. However, it is almost impossible for the position error signal to be zero due to the vibration of the magnetic disk or head and the circuits and apparatus in a hard disk drive. Thus, the position error signal value is changing according to a change of the position of the head.
When there is a defect in a written burst on a track, a problem occurs in that it is impossible to detect a position error signal from a burst region located next to the burst region having a defect. For solving the problem, two earlier methods are as follows.
One method is the maintenance of a servo defect map. This method is the same as the method treating a defective data sector when executing self-test in a hard disk drive. In other words, the microcontroller ignores a position error signal detected from a relevant servo sector by referring to a list of maintenance cylinders when the head is at the relevant servo sector after writing a servo sector repeatedly generating a high state of a position error signal on a maintenance cylinder.
The other method is to swap a track of a servo sector generating a high state of a position error signal for a surplus track.
For executing the two methods, it is necessary for the microcontroller to store a defective track, resulting in an occurrence of a problem of increased overhead of its capacity. The method of swapping a track has a problem in that normal servo sectors of the relevant track of a defective sector can not be used and a surplus track must be used instead.
The following patents each disclose features in common with the present invention but do not teach or suggest the specifically recited technique for controlling the position error signal of a disk drive in accordance with the present invention: U.S. Pat. No. 5,544,135 to Akin Jr. et al., entitled Fault Tolerant Servo Address Mark For Disk Drive, U.S. Pat. No. 5,592,348 to Strang Jr., entitled Method And Structure For Locating And Skipping Over Servo Bursts On A Magnetic Disk, U.S. Pat. No. 5,339,207 to Moon et al., entitled Servo System For Providing Increased Recording Density And Improved Operation Of The AGC Circuitry, U.S. Pat. No. 4,982,297 to Tsujisawa, entitled Head Positioning System For Compensating For Erroneous Detection Of Recorded Position Error Information, U.S. Pat. No. 4,910,617 to Brunnett et al., entitled Disk Drive Head Positioning Servo System Utilizing Encoded Track Zone Information, and U.S. Pat. No. 5,241,433 to Anderson et al., entitled Disk Drive Servo Control.